1. Field of the Invention
The present invention relates to polyester or polyether-polyester polyols. More particularly, the invention relates to polyester or polyether-polyester polyols prepared employing N-methylimidazole, triethylenediamine and/or triphenylphosphine as catalysts for the esterification step and mixtures of at least two of the following catalysts N-methylimidazole, triethylenediamine, triphenylphosphine and thiodialkylene glycol for the oxyalkylation step.
2. Description of the Prior Art
In order to prepare flame-resistant polyurethane foams, halogen- and ester group-containing polyether polyols are often used as the basic components.
According to DE-A 19 23 936 (U.S. Pat. Nos. 3,585,185); 3,459,733; 3,639,541; and 3,639,542; such polyether-polyester polyols are obtained by the reaction of polyether polyols, which in some cases may contain phosphorus in the initiator molecules, with halogen-containing carboxylic acid anhydrides, in particular tetrabromo- or tetrachlorophthalic acid anhydride, to form carboxylic acid half-esters and then through the subsequent oxyalkylation of the free carboxyl groups of said carboxylic acid half-esters.
In order to prepare the carboxylic acid half-esters, trialkylamines are used as catalysts in accordance with U.S. Pat. No. 4,144,395. This improves the color of the polyether-polyester polyols and shortens the reaction time.
The oxyalkylation is generally performed in the presence of either bases, such as alkali hydroxides or alkali alcoholates, or acids as catalysts. The disadvantage of this method is that the low selectivity of the catalysts means that both the oxyalkylation of the carboxyl groups and the addition of alkylene oxides to the resulting or already present hydroxyl groups of the polyester or polyether polyols are accelerated. In order to completely assure that all the carboxyl groups are fully esterified, the oxyalkylation step must be performed with a large access of alkylene oxides and a long reaction time. One further disadvantage is that the catalysts must be separated from the reaction mixture after completion of the reaction, which requires long and costly purification operations.
In order to avoid subsequent purification, as taught in EP-A 00468, the oxyalkylation of chlorine-containing phthalic acid half-esters is preferably performed in the absence of catalysts. However, with the non-catalyzed oxyalkylation a large excess of alkylene oxide is necessary to esterify all the carboxyl groups. A further disadvantage is that the alkylene oxides which have not reacted, up to 15 weight percent of the amount employed, must be distilled off and disposed of, which adversely affects both the economics of the process as well as its environmental impact.
The preparation of polyesters through the reaction of carboxylic acid anhydrides with alkylene oxides in the absence of water and in the presence of glycols and catalysts is described in U.S. Pat. No. 3,374,208. The catalysts cited here are metal compounds whose cations consist of zinc, tin, manganese, lead, nickel, cobalt, or cadmium ion, and anions of oxygen, chlorine, acetate, butyrate, phosphate, nitrate, stearate, oleate, and naphthenate ions.
The esterification of carboxylic acids with alkylene oxides in the presence of catalysts such as sulfuric acid, sodium acetate, iron (III)-chloride, etc. is also known (Methoden der organischen Chemie, in Vol. VIII, Houben-Weyl, Stuttgart: Georg Thieme Verlag, 1952. pp. 531-533).
In order to prepare polyurethanes from organic polyisocyanates and compounds having reactive hydrogen atoms, polyester or polyether-polyester polyols having highly reactive hydroxyl groups are required for various applications. Such compounds can be prepared through the introduction of primary hydroxyl groups by oxyalkylating conventional polyether polyols. However, to do this a large excess of ethylene oxide is required, leading to hydrophilic polyoxyethylene segments, which provide the resulting polyurethanes with hydrophilic properties. This hydrophilic characteristic is a disadvantage for important polyether polyol applications, for example, in the preparation of foams.
In order to avoid this disadvantage, the carboxylic acid half-esters as taught by European patent Application 82,111,822.1 are oxyalkylated with 1 mole of alkylene oxide per equivalent of carboxyl group in the presence of thiodialkyleneglycol as a catalyst. Particularly good results are obtained with this process when glutaric acid anhydride is used to prepare the carboxylic acid half-ester. However, if the polyols are reacted with other carboxylic acid anhydrides, such as tetrahydro- or phthalic acid anhydride, and the carboxylic acid half-esters are subsequently oxyalkylated, polyester or polyetherpolyester polyols are produced which, in part, are difficult to process into polyurethanes in a reproducible manner. An investigation into this problem revealed that there is a close relationship between the problems which occur and the formation of the carboxylic acid half-esters, whereby the formation of the carboxylic acid half-esters is poorer, when the hydroxyl number of the polyol is lower, in other words as the molecular weight is higher. The objective of the invention at hand was then to eliminate such process defects and to develop a process for the preparation of polyester or polyether-polyester polyols through catalytic esterification in which the carboxylic acid half-ester formation would proceed as quantitatively as possible and by which the subsequent oxyalkylation would not be negatively affected, but instead would occur as selectively as possible at the carboxyl group of the half-ester.